The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
A conventional transmission includes a pump assembly operable to circulate a hydraulic fluid, such as an oil, throughout the transmission to provide lubrication and/or cooling to the various components within the transmission. An exemplary pump assembly found in hybrid engine motor vehicles includes a primary pump, typically driven by the hybrid engine, and an auxiliary pump, typically driven by an electric motor. The main pump and auxiliary pump are in communication with a hydraulic fluid reservoir or sump. During operation, one of the pumps is activated while the other pump remains inactive or stationary. For example, when the main pump is active the auxiliary pump is stationary and hydraulic fluid is drawn from the sump through a main pump sump filter to the main pump where the hydraulic fluid is then delivered throughout the transmission. Excess hydraulic fluid not needed by the transmission is returned to the main pump via a main pump bypass circuit. When the auxiliary pump is activated the main pump may be stationary. When the main pump is stationary, hydraulic fluid is drawn from the sump through an auxiliary pump sump filter to the auxiliary pump where the hydraulic fluid is then delivered throughout the transmission. The small amount of excess hydraulic fluid not needed by the transmission is returned to the sump via the main pump bypass circuit. Typically, the main pump bypass circuit is not directly connected to the auxiliary feed circuit that connects the auxiliary pump to the sump.
During cold conditions, such as below 0 degrees Celsius, the hydraulic fluid from the main pump bypass circuit can damage the main pump sump filter if the main pump is stationary. This occurs as the hydraulic fluid forced by the running auxiliary pump enters the main pump bypass circuit and pressurizes the filter when the main pump is stationary. Additionally, when either the main pump or auxiliary pump are stationary, air can become entrapped within the main feed circuit and the auxiliary feed circuit near the inlet ports of the main pump and the auxiliary pump. Trapped air can create air lock where the pump is unable to provide the amount of hydraulic fluid flow required by the demands of the transmission. Accordingly, there is a need in the art for a pump system in a transmission that is usable in cold conditions, which is compact in size, and which reduces trapped air within the pumps.